A wide range of materials has been proposed for use in repairing hard tissues. For weight-bearing areas, prostheses which are capable of withstanding stress have ranged from metal rods to reconstituted animal bone. Various packing materials have also been used for augmentation of bony structures, such as the use of cross-linked collagen for alveolar ridge augmentation. It is desirable to have available a variety of materials suitable for the various types of skeletal repair, as each application has its unique set of parameters to determine the optimum implant. In addition, the physical handling properties of the material as it is manipulated by the medical practitioner is significant in permitting the practitioner to achieve a successful result, in part because the ease of manipulation determines the ability to succeed.
Attempts have been made to compose suitable materials of the chief organic and inorganic components of bone, namely, collagen and calcium phosphate mineral. Reports of attempts to use the collagen/mineral combination are numerous. For example, Lemons, J., et al., reported at the Second World Congress of Biomaterials in Washington, D.C., Apr. 27-May 1, 1984, on attempts to utilize collagen along with commercial hydroxyapatite and calcium phosphate to repair artificially created lesions in rabbits. The use of these mixtures did not result in reunion of the lesions. A control experiment using fresh autogenous bone, however, was successful in producing a union. Similarly, Levy, P., et al, J Periodontal (1981), 50:303-306, were unsuccessful in their attempts to utilize collagen/mineral gel implants to repair intra-bony defects in root canals of canine or monkey teeth. Gross, B. C., et al., Oral Surg (1980), 49:21-26, reported limited success in using mixtures of reconstituted lyophilized calfskin collagen in admixture with a hydroxyapatite preparation to induce bone growth through subperiosteal implants in monkeys. Various others have reported use of forms of collagen which clearly contain telopeptides, a major source of immunogenicity of collagen, in combination with minerals in bone repair. See, for example, Hayashi, K. et al., Arch Orthop Traumat Surg (1982) 99:265-269; Battista, U.S. Pat. No. 4,349,490 (using a hydrated gelatin); Cruz, Jr., U.S. Pat. No. 3,767,437 (using a calcium-precipitated form of collagen); and Battista, et al., U.S. Pat. No. 3,443,261 (utilizing, in addition to calcium phosphate, a "new form" of collagen which contains microcrystals of aggregated tropocollagen units.
Miyata, et al., U.S. Pat. No. 4,314,380, utilized a mineral backbone prepared directly by treatment of animal bone to remove all organic materials, which was then coated with an atelopeptide collagen. Japanese Application No. J58/058041, published Apr. 6, 1983, discloses a spongy porous calcium phosphate material having pores treated with atelopeptide collagen. The collagen derives from collagen-in-solution having a concentration of not more than 2% by weight. The Japanese application reports the advance of osteoblasts into the pores of the material and new bone growth. European Patent Application, Publication No. 030583, published June 24, 1981, discloses use of Collagenfleece.RTM. in admixture with hydroxyapatite in bone repair. This collagen material is a commercial product, is obtained from animal hide by proteolytic digestion, and is lyophilized and sterilized by gamma irradiation. This collagen preparation forms a soft membrane-like material but does contain telopeptides and is partially degraded by the processing.
EPO application Publication No. 164,483, published Dec. 18, 1985, discloses a process which is asserted to provide biocompatibility of a mineral/collagen mixture. In this mixture, solubilized collagen is cross-linked either in the presence of, or before the addition of, a calcium phosphate mineral component just to the point wherein it retains its resorbability and absorptive capacity with respect to body fluids, rather than permitting the cross-linking to proceed to completion. U.S. Pat. No. 4,516,276 to Mittelmeier discloses the combination of a nonfibrillar, nonreconstituted collagen along with hydroxyapatite.
U.S. patent application Ser. No. 848,443, filed Apr. 4, 1986, now U.S. Pat. No. 4,795,467 and its parent, U.S. Ser. No. 717,072, filed Mar. 28, 1985, both assigned to the same assignee as the application herein and incorporated by reference, disclose novel compositions containing reconstituted fibrillar atelopeptide collagen in admixture with a calcium phosphate mineral. Various methods are also disclosed for strengthening the composition, which methods include incubation of the mixture at specified temperatures and times, and the treatment of the dried mixture with heat. The preparation of the referenced applications, in order to be non-infective to treated subjects, must be prepared under aseptic conditions, as there is no provision in the disclosed procedures for direct sterilization. Typically, asceptic processing results in products with sterility assurance levels (i.e., probability of a non-sterile product unit) between 10.sup.-3 and 10.sup.-4.
The material which results after the various curing treatments disclosed in the above-referenced applications has a compressibility above 6 Newtons per square centimeter (N/cm.sup.2). Both this strength and further improvement in the compressibility indices are achievable by the curing processes disclosed therein.
The art offers no suitable composition for bone defect repair which is readily and efficiently sterilizable while retaining the efficient handling properties desired to permit effective insertion of the implant. The material should be resistant to compression, and yet sufficiently resilient to permit shaping into place, or alternatively, if to be used in a weight-bearing area, should be suitably rigid. The process and resulting product of the present invention remedies this omission in the art.
The invention takes advantage of an irradiation process which has previously been disclosed with regard to its impact on physical properties only in regard to preparations containing collagen alone. A summary of the effect of .gamma.-ray irradiation on collagen sutures, for example, is found in Artandi, Technical Report #149, Intl Atomic Energy Agency, Vienna, Manual on Radiation Sterilization of Medical & Biological Materials (1973) chap. 15, and a review of the effect of radiation on collagen as a tissue component is published by Bailey, A.J., in Internat Rev Connect Tis (1968), pp.233-281. In addition, PCT application No. WO81/00963 discloses that collagen materials can be increased in physical strength by heat treatment and by subjecting them to treatment with gaseous hydrogen halide. However, Applicant is aware of no disclosure in the art which shows the effect of .gamma.-ray irradiation on the physical properties and handling properties of collagen/mineral mixtures, although .gamma.-ray irradiation has been used to sterilize the lyophilized preparations disclosed in EPO publication No. 164,483 (supra) without further comment concerning either properties or further use.